[Xen-devel] [PATCH v2 13/27] x86/boot/64: Build head64.c as mcmodel large when PIE is enabled

[Thomas Garnier <thgarnie@xxxxxxxxxx>]

The __startup_64 function assumes all symbols have relocated addresses
instead of the current boot virtual address. PIE generated code favor
relative addresses making all virtual and physical address math incorrect.
If PIE is enabled, build head64.c as mcmodel large instead to ensure absolute
references on all memory access. Add a global __force_order variable required
when using a large model with read_cr* functions.

To build head64.c as mcmodel=large, disable the retpoline gcc flags.
This code is used at early boot and removed later, it doesn't need
retpoline mitigation.

Position Independent Executable (PIE) support will allow to extended the
KASLR randomization range below the -2G memory limit.

Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
---
 arch/x86/kernel/Makefile | 6 ++++++
 arch/x86/kernel/head64.c | 3 +++
 2 files changed, 9 insertions(+)

diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 29786c87e864..1ff6be34de66 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -22,6 +22,12 @@ CFLAGS_REMOVE_early_printk.o = -pg
 CFLAGS_REMOVE_head64.o = -pg
 endif
 
+ifdef CONFIG_X86_PIE
+# Remove PIE and retpoline flags that are incompatible with mcmodel=large
+CFLAGS_REMOVE_head64.o += -fPIE -mindirect-branch=thunk-extern 
-mindirect-branch-register
+CFLAGS_head64.o = -mcmodel=large
+endif
+
 KASAN_SANITIZE_head$(BITS).o                           := n
 KASAN_SANITIZE_dumpstack.o                             := n
 KASAN_SANITIZE_dumpstack_$(BITS).o                     := n
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 0c855deee165..2fe60e661227 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -64,6 +64,9 @@ EXPORT_SYMBOL(vmemmap_base);
 
 #define __head __section(.head.text)
 
+/* Required for read_cr3 when building as PIE */
+unsigned long __force_order;
+
 static void __head *fixup_pointer(void *ptr, unsigned long physaddr)
 {
        return ptr - (void *)_text + (void *)physaddr;
-- 
2.16.2.660.g709887971b-goog


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